Cracking in asphalt concrete surfaces is very common and has been a problem ever since the use of asphalt concrete began. With the advent of the asphalt laydown machine, to lay the hot mix asphalt concrete, came the longitudinal joint cracking. For surfaces which are not laid between curbs or forms, longitudinal joint cracking occurs. This problem has become accepted as an inevitable built-in flaw.
Of course, as soon as cracking develops, water can enter the cracks. When the water freezes it expands and causes further cracking and break-up of the asphalt concrete. Patching of the cracks is always necessary to prevent deterioration of the asphalt concrete surface. In warm climates, such as the Los Angeles Basin, which never experiences freeze-thaw there is a somewhat different but as bad a longitudinal cracking problem along these joints. The area next to the joint, not being properly compacted, has an excessively high volume of air voids. This causes the joint to break up much wider than just a simple crack. This type of cracking is much more difficult to repair than just a simple open crack. Eventually the asphalt concrete will usually be overlaid with new surfacing. This new overlay does not solve the problem for more than just a short time because the new overlay will crack above the old crack. This is a severe problem known as reflection cracking. Filling the cracks will only prolong the time of reflection cracking a short time. Reflection cracks are caused by vertical or horizontal movement in the pavements beneath the overlay. Filling the crack does not reinforce the crack but it can be reinforced with geotextile fabric. This fabric is usually not applied except when there is extensive cracking such as alligator and/or shrinkage cracking. In other words, the common practice is to just bury the longitudinal crack and then live with a new longitudinal crack. Cracking is discussed by Paul Schmidt in "Better Crack Repair", Pavement Maintenance, pp. 253-257 (September, 1991).
The weakest point of any asphalt pavement is generally along a construction joint, particularly the longitudinal paver joint. The density of the asphalt concrete is typically 2 to 5% lower at the longitudinal joint than in the remainder of the asphalt concrete.
There are many factors which can affect the quality and longevity of these joints. Cracks along joints occur primarily because of bond failure between adjacent asphalt concrete lanes. Many weak points are developed during construction through improper luting techniques, improper compaction, or improper paver overlap. Other weak areas are caused by the presence of dirt and debris or by the cold and possibly poor condition of the existing edge to be matched.
Any object expands in hot weather and contracts in cold weather. Consequently, conventional Portland cement concrete (PCC) parking lots and highways include expansion joints (usually with felt or other such material placed in the joints). When the (PCC) concrete expands, it compresses the material in the joints, thus relieving the tremendous internal stresses. Without such stress-relieving mechanisms, (PCC) concrete highways and parking lots can experience severe buckling and breaking.
On the other hand, asphalt concrete pavements are not built with expansion joints. They are flexible (non-rigid) pavements with an internal system of stress-relieving air voids. For most parking lots, compacted asphalt concrete includes air voids which constitute about 5 to 8% of the volume.
For airport projects, the Federal Aviation Administration now considers compacted pavement air voids so important that it will penalize contractors for too many voids (under-compaction) or too few voids (over-compaction).
Tennis courts and play areas often have wide thermal cracks that develop within 3 to 10 years. Contractors and producers generally prefer a very fine velvet-like surface for these applications. They therefore increase the dust and asphalt binder content on these projects, which in turn reduces the compacted pavement air voids.
In the long term this good intention can result in the formation of wide thermal cracks which are so common. The pavement's air voids are filled with dust and asphalt cement, thereby plugging the internal stress-relief mechanism. As a result, the pavement must form its own stress-relief system, namely a wide expansion joint or crack. The width of these cracks increases in cold weather and decreases in hot weather. Freezing, of course, causes the pavement to expand or heave.
Asphalt concrete is typically laid down by a paving machine which receives a bulk amount of heated asphalt concrete mixture (commonly known as hot mix asphalt) and then meters the mixture into an eight to twelve feet wide (or wider) onto the road base as the machine moves forwardly. For paving narrower lanes, a portion of an eight foot paver can be blocked off. There are also narrower walkway paving machines. This lane is also often referred to as a pass. Multiple lanes are laid side by side until the asphalt concrete mat has covered the entire width of the roadway or other surface to be covered.
Each lane is compacted by a very heavy roller machine which includes a large cylindrical drum or a plurality of closely spaced pneumatic tires, or a combination of both. The compacting machine is driven repeatedly over each lane of the asphalt concrete after it is laid to compact or consolidate the material. For the purpose of the present invention, the terms "compacting" and "consolidating" are used interchangeably to refer to the act of packing or compressing the asphalt concrete to a desired density. After one lane has been laid and then compacted, another lane is laid down adjacent to the longitudinal edge of the first lane, after which it is also compacted.
The main problem with conventional compacting techniques, in our opinion, is that there is nothing to hold the hot asphalt concrete mix material along the edge of the mat when it is being compacted or consolidated by the roller machine. As a result, the edge of the mat has a lower density (e.g., 2 to 5% lower). It is this reduced density along the longitudinal joint of adjoining lanes which ultimately leads to cracking between the lanes of the asphalt concrete mat over time which is longitudinal joint cracking.
To our knowledge, the only asphalt concrete roller which has included anything to compact the edge surface of an asphalt concrete mat is a machine sold by Bomag which included a small tapered wheel supported along one side of the large cylindrical packing drum. Apparently this wheel was provided primarily for making a more cosmetically acceptable edge surface to the mat. Because the wheel was mounted to the large roller with a solid mounting, no means were provided for applying a predetermined pressure to the edge of the mat. Consequently, when the large roller moved laterally with respect to the mat, the small wheel either applied no pressure to the edge or it applied a very large amount of pressure which can vary constantly between these two extremes.
Recently Bomag has shown in a publication an attachment to large vibratory tandems which is identified as an edge roller/cutter to hold the outer edge of an asphalt lift to reduce lateral expansion and increase compaction. The roller can be detached from the apparatus and replaced with a cutter wheel having a sharpened edge. The roller has a working surface which is disposed at an angle of 30.degree. off horizontal. The roller does not appear to be able to provide controlled uniform density of an asphalt lift.
Another manufacturer (Hamm) provides a side roller having a working surface which is angled at 45.degree. from horizontal. The roller includes a sharpened outer edge which can cut asphalt concrete. The lower edge of the roller is only two inches below the cylindrical packing drum.
The foregoing edge rollers do not include any means permitting lateral adjustment thereof. Further, the foregoing apparatus does not include means for adjusting the pressure exerted on the edge of an asphalt lift by the roller.
The other longitudinal cracking, edge cracking, can occur on the edge of any mat, be it a narrow walkway or a hundred foot wide highway. If the edge of the mat is not properly compacted to obtain uniform density, those outside few inches do not have the cohesive strength of the balance of the mat.
Although it is common practice to roll the edge surface of the roadway to round the edge, this is for cosmetic reasons and does not result in uniform compaction of the asphalt concrete edge.
There has not heretofore been provided an effective technique or apparatus for reducing or eliminating the formation of cracks in asphalt concrete surfaces.